The present invention relates generally to implantable intraluminal devices, particularly intraluminal grafts. Intraluminal stents are implanted in order to maintain luminal patency, typically after interventional methods have been employed to restore luminal patency from a diseased state, exclude an aneurysmal condition, bypass an occluded or obstructed anatomical region or to shunt body fluids. Surgically implantable prosthetics, particularly vascular prostheses, have been employed for many years. Expanded polytetrafluoroethylene (ePTFE) vascular grafts have been used as biocompatible implants for many years and the use of ePTFE as a bio-inert barrier material in intraluminal applications is well documented. Conventional ePTFE vascular grafts, however, typically lack sufficient diametric mechanical rigidity to maintain luminal patency in intraluminal applications.
Conventional externally supported ePTFE vascular grafts, such as the IMPRA Flex-Graft or the Gore Ring Graft, have an external beading of helically wound non-expanded or solid polytetrafluoroethylene, or of solid fluorinated ethylene-propylene co-polymer (FEP). Non-expanded or solid polytetrafluoroethylene is significantly more rigid than the ePTFE material due to its higher density and absence of interstitial voids. These externally supported ePTFE vascular grafts are not well-suited to interventional intraluminal procedures due to their inability to assume a reduced profile suitable for percutaneous delivery using a catheter and their inability to recover an enlarged diametric dimension in vivo.
Most intraluminal stents are formed of an open lattice fashioned either to be elastically deformable, such as in the case of self-expanding stainless steel spring stents, plastically deformable, such as in the case of balloon-expandable stainless steel PALMAZ stents, or thermally expandable such as by employing shape memory properties of the material used to form the stent. A common problem of most conventional intraluminal stents is re-occlusion of the vessel after stent placement. Tissue ingrowth and neointimal hyperplasia significantly reduces the open diameter of the treated lumen over time, requiring additional therapies.
The present invention makes advantageous use of the known biocompatible and material properties of ePTFE vascular grafts, and adds an abluminal supporting structure capable of being diametrically reduced to an intraluminal delivery profile and self-expanding in vivo to conform to the anatomical topography at the site of intraluminal implantation. More particularly, the present invention consists of an ePTFE substrate material, such as that described in U.S. application Ser. No. 08/794,871, filed Feb. 5, 1997, now U.S. Pat. No. 6,039,755, as a carrier for a helically wound, open cylindrical support structure made of a shape memory alloy.
The inventive intraluminal stent-graft device may be implanted either by percutaneous delivery using an appropriate delivery system, a cut-down procedure in which a surgical incision is made and the intraluminal device implanted through the surgical incision, or by laparoscopic or endoscopic delivery.
Shape memory alloys are a group of metal alloys which are characterized by an ability to return to a defined shape or size when subjected to certain thermal or stress conditions. Shape memory alloys are generally capable of being plastically deformed at a relatively low temperature and, upon exposure to a relatively higher temperature, return to the defined shape or size prior to the deformation. Shape memory alloys may be further defined as one that yields a thermoelastic martensite. A shape memory alloy which yields a thermoelastic martensite undergoes a martensitic transformation of a type that permits the alloy to be deformed by a twinning mechanism below the martensitic transformation temperature. The deformation is then reversed when the twinned structure reverts upon heating to the parent austenite phase. The austenite phase occurs when the material is at a low strain state and occurs at a given temperature. The martensite phase may be either temperature induced martensite (TIM) or stress-induced martensite (SIM).
When a shape memory material is stressed at a temperature above the start of martensite formation, denoted Ms, where the austenitic state is initially stable, but below the maximum temperature at which martensite formation can occur, denoted Md, the material first deforms elastically and when a critical stress is reached, it begins to transform by the formation of stress-induced martensite. Depending upon whether the temperature is above or below the start of austenite formation, denoted As, the behavior when the deforming stress is released differs. If the temperature is below As, the stress-induced martensite is stable, however, if the temperature is above As, the martensite is unstable and transforms back to austenite, with the sample returning to its original shape. U.S. Pat. Nos. 5,597,378, 5,067,957 and 4,665,906 disclose devices, including endoluminal stents, which are delivered in the stress-induced martensite phase of shape memory alloy and return to their pre-programmed shape by removal of the stress and transformation from stress-induced martensite to austenite.
Shape memory characteristics may be imparted to a shape memory alloy by heating the metal at a temperature above which the transformation from the martensite phase to the austenite phase is complete, i.e., a temperature above which the austenite phase is stable. The shape imparted to the metal during this heat treatment is the shape “remembered.” The heat treated metal is cooled to a temperature at which the martensite phase is stable, causing the austenite phase to transform to the martensite phase. The metal in the martensite phase is then plastically deformed, e.g., to facilitate its delivery into a patient's body. Subsequent heating of the deformed martensite phase to a temperature above the martensite to austenite transformation temperature, e.g., body temperature, causes the deformed martensite phase to transform to the austenite phase and during this phase transformation the metal reverts back to its original shape.
The term “shape memory” is used in the art to describe the property of an elastic material to recover a pre-programmed shape after deformation of a shape memory alloy in its martensitic phase and exposing the alloy to a temperature excursion through its austenite transformation temperature, at which temperature the alloy begins to revert to the austenite phase and recover its preprogrammed shape. The term “pseudoelasticity” is used to describe a property of shape memory alloys where the alloy is stressed at a temperature above the transformation temperature of the alloy and stress-induced martensite is formed above the normal martensite formation temperature. Because it has been formed above its normal temperature, stress-induced martensite reverts immediately to undeformed austenite as soon as the stress is removed provided the temperature remains above the transformation temperature.
The present invention employs a wire member made of either a shape memory alloy, preferably a nickel-titanium alloy known as NITINOL, spring stainless steel or other elastic metal or plastic alloys, or composite material, such as carbon fiber. It is preferable that the wire member have either a generally circular, semi-circular, triangular or quadrilateral transverse cross-sectional profile. Where a shape memory alloy material is employed, pre-programmed shape memory is imparted to the wire member by helically winding the wire member about a cylindrical programming mandrel having an outer diametric dimension substantially the same, preferably within a tolerance of about +0 to −15%, as the ePTFE substrate and annealing the programming mandrel and the wire member at a temperature and for a time sufficient to impart the desired shape memory to the wire member. After annealing, the wire member is removed from the programming mandrel, straightened and helically wound about the abluminal wall surface of an ePTFE tubular member at a temperature below the As of the shape memory alloy used to form the wire member.
In order to facilitate bonding of the wire member to the ePTFE tubular member, it is preferable that a bonding agent capable of bonding the support wire member to the ePTFE tubular member be used at the interface between the wire member and the ePTFE tubular member. Suitable biocompatible bonding agents may be selected from the group consisting of polytetrafluoroethylene, polyurethane, polyethylene, polypropylene, polyamides, polyimides, polyesters, polypropylenes, polyethylenes, polyfluoroethylenes, silicone fluorinated polyolefins, fluorinated ethylene/propylene copolymer, perfluoroalkoxy fluorocarbon, ethylene/tetrafluoroethylene copolymer, and polyvinylpyrolidone. The bonding agent may constitute an interfacial layer intermediate the wire member and the ePTFE tubular member, or may be a polymeric cladding at least partially concentrically surrounding the wire member.
Where a cladding is provided, the cladding is preferably a polymeric material selected from the group consisting of polytetrafluoroethylene, polyurethane, polyethylene, polypropylene, polyamides, polyimides, polyesters, polypropylenes, polyethylenes, polyfluoroethylenes, silicone fluorinated polyolefins, fluorinated ethylene/propylene copolymer, perfluoroalkoxy fluorocarbon, ethylene/tetrafluoroethylene copolymer, and polyvinylpyrolidone. The cladding may be either co-extruded with the wire member, extruded as a tube into which the wire member is concentrically inserted after annealing the wire member, or provided as an elongate member which a longitudinal recess which co-axially receives the wire member. Where the bonding agent employed is a melt thermoplastic which has a melt point below the crystalline melt point of polytetrafluoroethylene, the melt thermoplastic bonding agent and the wire member are wound about the ePTFE tubular member, and constrained thereupon, such as by application of circumferential pressure, then the assembly is then exposed to the melt temperatures without longitudinally supporting the assembly.
However, where the bonding agent is polytetrafluoroethylene, bonding of the wire member to the ePTFE tubular member requires exposing the assembly to temperatures above the crystalline melt point of polytetrafluoroethylene in order to effectuate bonding of the wire member to the ePTFE. This is preferably accomplished by introducing the assembly into a sintering oven while the assembly is on a mandrel and the assembly secured to the mandrel by an external helical wrapping of TEFLON tape applied to opposing ends of the assembly to longitudinally constrain the assembly and reduce or eliminate the tendency of the assembly to longitudinally foreshorten during sintering.